The following account of the prior art relates to one of the areas of application of the present invention, namely that of the hearing aid industry. In hearing aids it is customary to provide a telecoil, which is responsive primarily to changes in the magnetic flux in the surroundings. Such a telecoil is responsive eg to the magnetic flux changes induced by the small loudspeakers in telephone handsets, and this allows hearing aid users to speak on the phone while receiving the audio signal from the telephone via the telecoil, which will aid the hearing aid user when speaking on the phone. Further, there is now provided tele-loop installations at many public areas such as in churches, concert halls, movie theatres and also in private homes such installations has been in use for a long time. When a hearing aid user has a hearing aid with a telecoil antenna, the user may then receive the signal transmitted via the teleloop installation in the area and this is often a better and clearer signal, than is the signal provided from the microphones of the hearing aid. It has been a long known problem, that the telecoil will inevitably also respond to the magnetic signal leaked from the nearby speaker (called a receiver) of the hearing aid itself, and this will lead to a very annoying noise for the user. It has been the customary practice to enclose the receiver in a metal housing, composed of a metal composition known as mu-metal or μ-metal. This material will effectively isolate the receiver and the coil within the receiver from the telecoil. However this is a rather expensive procedure, and valuable real estate within the hearing aid is used which makes the hearing aid look bulky and less attractive to the user. Alternatively various shields or screens of mu-metal have been used, but such screens are difficult to insert in a hearing aid constructions without making fundamental changes in constructional details of the hearing aid, especially so, when the hearing aid comprises a hermetically closed chamber for the receiver.